Greenhouse gas has been emitted by the use of fossil fuel, thus causing global warming. Such global warming results in climate change and environmental change, and jeopardizes the survival of all organisms including humans. Therefore, many studies and developments for reducing carbon dioxide have been performed. As one method, a study has been conducted actively for recovery and biological conversion of carbon dioxide.
A study has been conducted actively for microalgae as photosynthetic organisms for the biological conversion of carbon dioxide. Phytoplankton microalgae use the sun as energy source, similarly to other photosynthetic organisms, and grow through photosynthesis for immobilizing carbon dioxide.
The reasons why microalgae draw attention as means for immobilizing carbon dioxide are as follows. First, since microalgae harness solar energy as a main energy source, similarly to carbon dioxide absorption in a plant, microalgae need only a small amount of energy for recovering carbon dioxide. Therefore, since the small amount of carbon dioxide is produced at the time of operation of immobilizing carbon dioxide, removal efficiency is high from the viewpoint of a carbon dioxide resin.
Second, microalgae have the immobilization rate of carbon dioxide higher than that of a plant, and a required site area is small. According to the findings of the Tokyo Electric Power Research Institute, the immobilization rate of microalgae is 2.8 times higher than the fastest growing sugar cane, and 15 times higher than of the most common species of pine in Korea.
In addition, there is no need for separation and concentration of carbon dioxide because carbon dioxide can be immobilized directly from combustion gas. In addition, microalgae which are produced during immobilization of carbon dioxide may be used as biological products because they contain various useful substances.
However, when the carbon dioxide-immobilization process using microalgae is performed by using the bioreactor which has been applied practically in industries, it is difficult to reduce energy and supply light energy for allowing microalgae to grow, due to high consumption of electrical energy.
Generally, an apparatus for culturing photosynthetic organisms for the purpose of carbon dioxide immobilization is usually divided into an open-type culture apparatus for outdoor mass culture and a close-type photobioreactor having a small volume. In the case of the open-type outdoor culture apparatus, it was usually used in a form such as a lake or a large pond in Germany, Japan or U.S. However, since the open-type outdoor mass culture apparatus in a form of a pond should be manufactured by an expensive reinforced concrete structure, a lot of energy is consumed at the time of consecutive stirring, and pollution prevention, and separation and purification of cultured microalgae are difficult. Furthermore, in the case of the mass culture apparatus, it has slow growth rate of photosynthetic organisms and low growth yield because generally light is not effectively transferred to the inner portion.
Presently, the developed close-type photobioreactor includes a general stirring type reactor, a plate-type reactor, a tube-type reactor, and a column-type reactor, and the like. It could be expected that the close-type reactor has a cell growth rate higher than that of the outdoor mass culture apparatus, and it is easy to control operation conditions. However, the close-type reactor has a high initial cost and a high operation management cost, and it is difficult to use efficiently light energy as a crucial factor of a photobioreactor. In order to use light energy, a reactor in which a light source is installed in the reactor was developed. However, the reactor has good light efficiency, but it is not efficient because electrical energy of artificial fluorescent lamp or LED is used. In addition, since the close-type reactor is generally manufactured by a reinforced glass or acryl which is stationary, it is difficult to perform mass culture and clean the reactor, and indoor space cannot be employed effectively.
The present inventors found that, when a reaction vessel where photosynthesis occurs is made of a transparent film, instead of a reinforced glass or acryl that has been generally used, the reaction vessel has good light transmittance, thus enabling microalgae to grow well, and the reaction vessel has good mobility because it is light, thus enabling the economically advantageous manufacture and operation thereof. Also, the present inventors found that, when the inner space of a photobioreactor made of a transparent film is partially partitioned, carbon dioxide and photosynthetic organisms are further dispersed therein and reduction of light transmittance caused by the deformation of the reactor may be prevented. The present invention was accomplished.